[time-nuts] OCXO noise as they retrace

[Bob Stewart]

Hi Attila et al,
I think I've finally got a handle on the problem.  A few months ago I posted that I was getting phase pops on my 5370A.  At least I thought it was on the 5370.  So, I bought another one on ebay, and the problem disappeared.  Then it came back, and rapidly got worse.  Unfortunately, the OCXO I had put into one of my GPSDOs was also bad.  So, when I did a three-way comparison, no comparison looked good.  Thus my conclusion that the new 5370 had somehow gone bad in the same way that the old one did.
So, last night, I plugged in an older unit that had always tested good and let it cook overnight.  This morning, comparing that to the PRS got me about 1E-10 --- not very good.  Then I compared the old one to what I had considered the best of the new units I have on hand, and it's down in the 4E-11 or so range at 1 second.  So, I swapped out the OCXO in the other unit I had been testing, and the ADEV at 1s is just under 4E-11, which is where I would expect with my test equipment.

So, my conclusion is that the PRS was the cause of the popping I reported back around February, but it improved for awhile before getting worse: about 1E-10 at 1 second tau.  At the same time, one of the units I was testing not only had a noisy OCXO, but the previous one I had in that unit was also bad.  So, essentially this was a comedy of errors.
I'm left with using one of my GPSDOs as a reference.  But, the difference between good and bad seems to be large enough that I think it will be good enough, even if it eventually were to become noisy.

I do use ebay OCXOs in my GPSDOs to keep the cost way down.  This experience underlines how important it is to test them carefully before depending on them.  Out of about 30 tested, I've had maybe 10 which are unacceptably noisy, which is more or less what I had planned on.  Like Bob Camp says, you never know what you're getting when you get one of these Chinese "recycled" OCXOs.  But, if you buy in some quantity, and carefully test them, enough are good to make them viable.
Bob
 -----------------------------------------------------------------------------------
GFS GPSDO list:
groups.yahoo.com/neo/groups/GFS-GPSDOs/info

      From: Attila Kinali <attila at kinali.ch>
 To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com> 
 Sent: Wednesday, June 29, 2016 10:49 AM
 Subject: Re: [time-nuts] OCXO noise as they retrace
   
On Mon, 27 Jun 2016 21:55:55 +0000 (UTC)
Bob Stewart <bob at evoria.net> wrote:

> I've had a specific GPSDO running for some time now, and I notice that
> the noise at tau 1s has gotten worse as the retrace flattens out.
>  In this case, the  ADEV was about 3.6E-11 a month or so ago, and has now
> gone up to about 8.5E-11.  (Measurements performed by a 5370A against a
> PRS-45A Cs standard.)  Is this normal as the startup drift settles out?
>  It's been on the same power supply module during this time, but I have been
> using it to test new code, so the DAC has been cycled from midpoint to lock
> numerous times.  Another unit that's been running for some time has done
> essentially the same thing.
>
> Weather?  Environment?  GPS demons?

As the change is just about a factor of 2, my first guess would be that
you see numerical effects due to the finite tuning resolution of the GPSDO.
You can see it as a similar effect as in delta-sigma modulators. As long
as the output value is changing, the noise power (which can be assumed
to be constant for a first order approximation) will be spread over a
wide frequency range. But, if you output a constant value, the noise power
will get concentrated in a couple of spurs, which will then stick out quite
a bit.

A simple test for this hypothesis would be to send the GPSDO into hold-over
mode and measure the ADEV of the OCXO again with constant EFC voltage.
Another indication would be, if the ADEV increased only in a narrow range,
while slightly decreasing overall (though, this is a much weaker argument
as it can be confunded by other phenomena).


            Attila Kinali
-- 
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